Location-based services and emergency call positioning drive the development of positioning in wireless networks. Positioning support in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) was introduced in Release 9. This enables operators to retrieve position information for location-based services and to meet regulatory emergency call positioning requirements. Global Positioning System (GPS)-enabled terminals can meet the requirement for positioning, but they cannot provide the required availability due to the satellite signals being blocked in urban and indoor environments. Therefore, other techniques are needed in such environments.
Positioning in LTE is supported by the architecture illustrated in FIG. 1, with direct interactions between a User Equipment device (UE) and an Enhanced Serving Mobile Location Center (E-SMLC), which may also be referred to as a location server, are via the LTE Positioning Protocol (LPP). Moreover, there are also interactions between the E-SMLC and an enhanced or evolved Node B (eNB) via the LPP Annex (LPPa) protocol, to some extent supported by interactions between the eNB and the UE via the Radio Resource Control (RRC) protocol.
The following positioning techniques are considered in LTE (see, for example, 3GPP Technical Specification (TS) 36.305 v12.0.0):                Enhanced Cell Identifier/Identity (ID). Essentially, cell ID information associates the UE to the serving area of a serving cell, and then additional information determines a finer granularity position.        Assisted Global Navigation Satellite System (GNSS). GNSS information retrieved by the UE is supported by assistance information provided to the UE from the E-SMLC.        Observed Time Difference of Arrival (OTDOA). The UE estimates the time difference of reference signals from different base stations and sends this information to the E-SMLC for multilateration.        Uplink Time Difference of Arrival (UTDOA): The UE is requested to transmit a specific waveform that is detected by multiple location measurement units (e.g., an eNB) at known positions. These measurements are forwarded to the E-SMLC for multilateration.        
OTDOA is a UE-assisted method in which the UE measures the Time of Arrival (TOA) of specific Positioning Reference Signals (PRSs) from multiple eNBs and computes the relative differences. These Reference Signal Time Difference (RSTD) measurements are quantized and reported via LPP to the E-SMLC together with an accuracy assessment. Based on known positions of the eNBs and their mutual time synchronization, the E-SMLC estimates the UE position from the RSTD measurements and covariance reports using multilateration. The accuracy depends on the radio conditions of the received signals, the number of received signals, as well as the deployment, which means that it will vary spatially.
FIG. 2 illustrates multilateration in OTDOA while considering eNB1 as the reference cell. The TOAs from neighboring cells are subtracted from that of the reference cell to provide corresponding RSTD measurements. Each such RSTD measurement determines a hyperbola, and the intersecting point of these hyperbolas can be considered as the UE position.
In principle, it is possible to measure RSTD on any downlink signals e.g., Cell-Specific Reference Signals (CRSs). However, in OTDOA, the UE is required to detect multiple neighboring cell signals, but these signals suffer from poor hearability. Hence, PRSs have been introduced to improve OTDOA positioning performance. FIGS. 3 and 4 show the arrangement of the PRSs (i.e., the PRS patterns) in one resource block for normal Cyclic Prefix (CP) and extended CP, respectively. In such a PRS subframe, in order to reduce the interference with neighboring cells, no Physical Downlink Shared Channel (PDSCH) data is transmitted. The Physical Downlink Control Channel (PDCCH) and CRSs are retained in the subframe, while PRSs are distributed in a “diagonal” way in between CRSs. Similar to CRS, a cell-specific frequency shift (defined as Physical Cell Identity (PCI) modulo 6) is applied to the PRS pattern, which helps avoid time-frequency PRS collision for up to six neighbor cells. Mathematically, according to 3GPP TS 36.211 V13.0.0, the PRSs are mapped to the resource element (k, l), i.e. the time-frequency PRS pattern can be written as:
for normal CP:
                    ⁢          k      =                        6          ⁢                      (                          m              +                              N                RB                DL                            -                              N                RB                PRS                                      )                          +                              (                          6              -              l              -                              v                shift                                      )                    ⁢          mod          ⁢                                          ⁢          6                          l    =          {                                                                                                        3                    ,                    5                    ,                    6                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                    0                                                                                                                    1                    ,                    2                    ,                    3                    ,                    5                    ,                    6                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                                          1                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                                              (                                                  1                          ⁢                                                                                                          ⁢                          or                          ⁢                                                                                                          ⁢                          PBCH                          ⁢                                                                                                          ⁢                          antenna                          ⁢                                                                                                          ⁢                          ports                                                )                                                                                                                                                              2                    ,                    3                    ,                    5                    ,                    6                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                                          1                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                                              (                                                  4                          ⁢                                                                                                          ⁢                          PBCH                          ⁢                                                                                                          ⁢                          antenna                          ⁢                                                                                                          ⁢                          ports                                                )                                                                                                                  ⁢                                                  ⁢                                                  ⁢            m                    =          0                ,        1        ,        …        ⁢                                  ,                                            2              ·                              N                RB                PRS                                      -                          1              ⁢                                                          ⁢                                                          ⁢                              m                ′                                              =                      m            +                          N              RB                                                ma                  ⁢                                                                          ⁢                  x                                ,                DL                                      -                          N              RB              PRS                                          where ns is the slot number and NRBPRS, and NRBDL, and NRBmax,DL denote the number of Resource Blocks (RBs) for PRS, PDSCH, and downlink maximum bandwidth, respectively; and
for extended CP:
                    ⁢          k      =                        6          ⁢                      (                          m              +                              N                RB                DL                            -                              N                RB                PRS                                      )                          +                              (                          5              -              l              -                              v                shift                                      )                    ⁢          mod          ⁢                                          ⁢          6                          l    =          {                                                                                                        4                    ,                    5                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                    0                                                                                                                    1                    ,                    2                    ,                    4                    ,                    5                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                                          1                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                                              (                                                  1                          ⁢                                                                                                          ⁢                          or                          ⁢                                                                                                          ⁢                          2                          ⁢                                                                                                          ⁢                          PBCH                          ⁢                                                                                                          ⁢                          antenna                          ⁢                                                                                                          ⁢                          ports                                                )                                                                                                                                                              2                    ,                    4                    ,                    5                                                                                                              if                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                      mod                      ⁢                                                                                          ⁢                      2                                        =                                          1                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                                              (                                                  4                          ⁢                                                                                                          ⁢                          PBCH                          ⁢                                                                                                          ⁢                          antenna                          ⁢                                                                                                          ⁢                          ports                                                )                                                                                                                  ⁢                                                  ⁢                                                  ⁢            m                    =          0                ,        1        ,        …        ⁢                                  ,                                            2              ·                              N                RB                PRS                                      -                          1              ⁢                                                          ⁢                                                          ⁢                              m                ′                                              =                      m            +                          N              RB                                                ma                  ⁢                                                                          ⁢                  x                                ,                DL                                      -                                          N                RB                PRS                            .                                          The bandwidth for PRBPRS, is configured by higher layers and the cell-specific frequency shift is given by vshift=NIDcell mod 6.
In an LTE system, consecutive PRS subframes (a.k.a. positioning occasions) are transmitted periodically in the downlink. In other words, PRSs are be transmitted in NPRS consecutive downlink subframes, where NPRS is configured by higher layers. According to the LTE specifications, one positioning occasion may contain up to six consecutive PRS subframes. The period of one positioning occasion can be configured to every TPRS=160, 320, 640, and 1280 milliseconds (ms). It is noted that, in Time Division Duplexing (TDD) mode, uplink subframes and other special frames cannot contain PRSs. Another parameter to characterize the PRS transmission schedule is the cell-specific subframe offset, which defines the starting subframe of PRS transmission relative to System Frame Number (SFN)=0. As shown in Table 1 below (which is reproduced from 3GPP TS 36.211), the PRS periodicity TPRS and subframe offset ΔPRS are derived from the PRS configuration index IPRS. The PRS configuration index IPRS is configured by higher layers. PRSs are transmitted only in configured downlink subframes. PRSs are not transmitted in Downlink Pilot Time Slots (DwPTSs). The PRS instances, for the first subframe of the NPRS downlink subframes, must satisfy (10×nf+└ns/2┘−ΔPRS)modTPRS=0.
TABLE 1Positioning reference signal subframe configurationPRS periodicityPRS subframe offsetPRS configuration IndexTPRSΔPRSIPRS(subframes)(subframes) 0-159160IPRS160-479320IPRS − 160 480-1119640IPRS − 4801120-23991280 IPRS − 11202400-4095Reserved
In some cases, in particular in a dense deployment, using only a cell-specific frequency shift may not be sufficient to avoid interference from neighboring cells. Therefore, PRS muting has been introduced to further reduce inter-cell interference by muting PRS transmissions in other cells based on a periodic “muting pattern.”
The positioning architecture in LTE operates via two positioning protocols: the LPP and the LPPa. LPP is used for communication between the E-SMLC and a UE, while LPPa is the communication protocol between an eNB and the E-SMLC. The information exchange between an eNB and an E-SMLC, as specified in 3GPP TS 36.455 V13.0.0, may be used for OTDOA positioning to support data collection from eNBs and configurations to eNBs. Such configuration information includes PRS configuration index table, number of consecutive PRSs, PRS bandwidth, etc.
In regard to RSTD reporting, the UE estimates the TOA of a reference cell and other detected cells based on received assistance information from the E-SMLC. Then, the UE computes the RSTD of each detected cell in relation to the reference cell. The RSTD measurements are subject to a quantization with a resolution of 1 Ts for RSTD measurement within ±4096 Ts, and 5 Ts otherwise (1 Ts=1/(15000×2048) seconds is the LTE basic time unit) (see 3GPP TS 36.133, Section 9.1.10.3).
In addition, the UE estimates the RSTD measurement quality and reports the uncertainty via a range:[nR,(n+1)R−1],where the reporting resolution is R={5, 10, 20, 30} meters, and n is an index to indicate the value range within which the RSTD uncertainty is estimated to be (see 3GPP TS 36.355, Section 6.5.1.5).
Current PRS introduced in LTE Release 9 was designed for macro base stations to support mainly outdoor positioning. As one type of reference signal, the generation of PRSs is associated with PCI and, therefore, PCI is the label to differentiate the PRSs from neighboring cells. Ongoing enhancements to the United States (US) Federal Communication Commission (FCC) Enhanced 911 capability are focusing on in-building positioning. For indoor small-cell scenarios, low-power Remote Radio Heads (RRHs) or distributed antennas are popularly deployed to enhance the spectral efficiency. However, those Transmit Points (TPs) belonging to the same eNB share the same PCI. As defined in 3GPP TS 36.455 V12.2.0, the “E-UTRAN Access Point Position” is associated with PCI, i.e., the locations of TPs sharing the same PCI cannot be differentiated. Hence, they cannot be used for indoor positioning based on OTDOA.